Polyvinyl chloride (PVC) compositions are commonly used as packaging materials for cosmetics, pharmaceuticals, foods and health care products. Polyvinyl chloride is preferentially selected for these packaging applications due to its low cost, clarity and ease of processing. In some packaging applications it is very desirable that plastic materials exhibit a very high gas impermeability. The gas barrier properties of PVC compositions are adequate for some applications but require improvement for the protection of extremely oxygen-sensitive materials.
The gas barrier properties of PVC may be improved by chemical or physical modification, or by the addition of a filler or a protective coating. A preferred approach to improving the gas barrier properties of PVC is by blending with barrier materials. Blends in general do not always have the additive properties of the two components. The properties of the blend in some instances are worse than the properties of the pure components. Therefore, blending a barrier polymer with PVC polymers does not necessarily lead to a composite with improved gas impermeability relative to PVC.
Packaging materials based on copolymers of ethylene and vinyl alcohol (EVOH) are excellent barriers to oxygen at low relative humidity, but are rather poor barriers at higher humidity conditions. One method for overcoming this problem is to blend the EVOH copolymer with a polymer that is primarily hydrophobic such as polyvinyl chloride.
U.S. Pat. No. 4,003,963 describes vinyl chloride polymer barrier packaging compositions which are blends of vinyl chloride polymers containing no carboxyl groups and about 20 to 30% by weight of an ethylene-vinyl alcohol copolymer containing greater than 50 mole percent of vinyl alcohol. The compositions are reported to have enhanced resistance to gas permeability and moisture vapor transmission.
The addition of ethylene-vinyl alcohol copolymers to polyvinyl chloride can produce a transparent compound with a permeability to oxygen that is lower than a polyvinyl chloride itself. However, when blends of PVC and EVOH polymers are melt blended on chrome surfaced compounding equipment, the highly polar EVOH preferentially adheres to the metallic surface. The "loss" of EVOH to the chrome surface is substantial and can make quantitative control of additive levels difficult. Additionally the EVOH which adheres to the chrome must be removed between batches thereby increasing the cost of the blending process.